Award: PLR-1142097

The Ross Sea plays a major role in the transfer of organic carbon from the surface into the deep sea due to the combination of high seasonal productivity and Antarctic bottom water formation. This part of the collaborative project examined the abundances and relative distribution of macroscopic particles (larger than 100 micrometers) in the Ross Sea late in the season February - March when the Ross begins to freeze over.The measurements were based on a combined deployment of a video particle profiler (VPP) and a high-resolution digital holographic microscope (DIHM) custom-built for deep-sea deployments. Long-distance (100s of kilometers) and short-distance (10s of kilometers) sections showed high variability of particle distributions that co-varied with the density structure of the water column. Particle export was apparent at sites of locally weakened pycnoclines, likely an indirect effect of nutrient mixing into the surface layer and local blooms that lead to export. Particle volume abundances at 200-300 m depth were highly correlated with particle volume abundances in the upper mixed layer (<60 m), consistent with particles at depth primarily the result of export rather than lateral advection. A dominant group of phytoplankton is the haptophyte Phaeocystis antarctica which forms globular colonies. With the digital inline holographic camera we were able to determine the vertical distribution of these colonies at very high resolution (i.e., in meter bins). At the beginning of our expedition, Phaeocystis antarctica were retained in the upper mixed layer but sank below the euphotic zone within a period of two weeks. Fine-scale analysis at a resolution smaller than1 m revealed a highly patchy environment in all casts and the degree of this patchiness depended on the depth layer and the location in the Ross Sea. Patchiness, as determined by the Lloyd index of patchiness and the Index of Aggregation, increased in and below the pycnocline presumably due to aggregation of particles while accumulating on density gradients. In contrast, particles in the upper mixed layer and in the nepheloid layers (i.e., the layers of high particle abundance close to the ocean floor) were more randomly distributed. In 40 of the 84 VPP depth profiles, a periodicity of particle peaks ranged from 10 to 90 m with a mode of 30 m, which can be regarded as the "relevant scale" or "characteristic patch size" of the vertical distribution of particles. While chlorophyll fluorescence and particle mass determined by VPP were significantly correlated at higher particle abundances, the relationship changed from cast to cast, reflecting changes in the relative contribution of fresh phytoplankton to total particle mass. Particles that sank below the main pycnocline were composed of phytoplankton, marine snow with and without embedded phytoplankton, crustacean plankton, and a surprisingly high percentage of heterotrophic (and perhaps mixotrophic) protists, such as acantharians and tintinnids. This project contributed significantly to undergraduate and graduate student education at Old Dominion University. Outreach efforts included collaboration with the Virginia Aquarium, tours of our facilities to the general public, and demonstrations and workshops for middle-school students and their mentors. In these sessions, students were able to lay their hands on oceanographic equipment, and performed their own reconstruction of organisms from holographic images originally collected in the seas surrounding Antarctica. Last Modified: 01/19/2017 Submitted by: Alexander B Bochdansky